1. Field of the Invention
The present invention relates to a dynamic pressure bearing having dynamic pressure generating grooves on at least one of its shaft member or its sleeve member receiving the shaft member inside thereof.
2. Description of the Prior Art
FIG. 1 shows a prior art dynamic pressure bearing having a shaft 11 and a sleeve 12. On the outer peripheral surface of the shaft 11 there are provided first oblique grooves 11a which are oblique in one direction with respect to a generating line of a cylindrical surface of the outer peripheral surface toward the rotating direction X, and second oblique grooves 11b which are spaced from the first oblique grooves 11a in the axial direction and which are oblique in a direction opposite to the direction in which the first oblique grooves 11a are oblique. The first oblique grooves 11a and the second oblique grooves 11b constitute herringbone type dynamic pressure generating grooves. A clearance between the sleeve 12 and the shaft 11 is filled with a lubricating fluid. When the shaft 11 is rotated in the rotating direction X, dynamic pressure is generated in the clearance between the shaft 11 and the sleeve 12 by the dynamic pressure generating grooves comprised of the first and second oblique grooves 11a and 11b. As a result, the shaft 11 is radially supported in the sleeve 12.
In the above conventional dynamic pressure bearing, the dynamic pressure generated in an intermediate cylindrical zone 17 between a cylindrical zone 15 where the first oblique grooves 11a are present and another cylindrical zone 16 where the second oblique grooves 11b are present is generally uniformly distributed in the axial direction, so that a bearing load and a bending moment can be supported over a wide range in the axial direction. Accordingly, the dynamic pressure bearing, even of a short bearing span, can have an increased load-supporting capacity, compared with bearings having V-shaped dynamic pressure generating grooves. Therefore, the whirl of the shaft 11 can be reduced.
However, the above-described conventional dynamic pressure bearing does not have a function of retaining the lubricating fluid in the intermediate zone 17 at the time of start-up or stoppage of the operation at which the rotating speed of the shaft is too low to cause the dynamic pressure to be generated. Therefore, when there is no or insufficient dynamic pressure, the outer peripheral surface of the shaft 11 and the inner peripheral surface of the sleeve 12 are brought into direct contact with each other in the intermediate zone 17. As a result, the contact surfaces of the shaft 11 and the sleeve 12 wear. Also when the shaft 11 of the conventional dynamic pressure bearing is erroneously rotated in reverse, the dynamic pressure generating grooves act reversely, so that the lubricating fluid flows out from between the shaft 11 and the sleeve 12. This results in deteriorated lubrication, which causes the contact surfaces of the shaft 11 and the sleeve 12 to seize up.